It is such a difficult time - you have my sympathy. We had to make the decision for an aunt, nearly 87 with pretty bad dementia. She was refusing to eat and drink after the umpteenth UTI and the care home asked whether we wanted her taken to hospital and put on a drip, or (basically) left at the care home to die.
We could only ask the GP what he'd do if it were his much-loved aunt, and he said he'd leave her where she was, in familiar surroundings, where they would keep her comfortable, rather than take her to a noisy hospital, to be poked about by strangers, when it was only going to happen again quite soon.
I sat with her a lot during her last days. The care home and staff were lovely - the staff did keep trying with sips of water and spoons of yoghurt, but she clearly did not want any of it -she would close her mouth and turn her head away. I think she had honestly just had enough. She did not seem to be in any discomfort and drifted away quite peacefully. It was a horrible decision at the time, but I never regretted it afterwards - I am sure it is what she'd have wanted.
In such cases I have heard it put (by a doctor) like this: 'They are not dying because they are not eating and drinking. They are not eating or drinking because they are dying.'
When it came to my mother some years later, there had already been a discussion with the care home about end of life. Hospitals are often terrible and frightening places for people with dementia, when they do not understand what is going on, or why, and are all too liable to try to pull drips etc. out. So the CH knew that she was to go to hospital only if absolutely necessary - e.g.in the case of another broken hip - she'd already had one at over 90. There was already a DNR in place, which IMO was both sensible and kind for someone already well over 90with advanced dementia.
In the end she went downhill quite suddenly at 97 and died within 36 hours, and the question of hospital never arose.